One of my previous boyfriends had a gf when we first met, something which he conveniently 'forgot' to tell me. When he did tell me about it, he claimed he and his gf were about to break up and it was just a matter of when it was going to happen. It turned out it wasn't quite that simple and the whole thing got rather messy. We did end up having a relationship for quite a long time, but all the drama in the beginning ruined a lot for us and hurt his ex a whole lot and it's not something I ever want to go through again.